vm: Fix anonymous memory clustering under ASLR
By default, our ASLR implementation is supposed to cluster anonymous memory allocations, unless the application's mmap(..., MAP_ANON, ...) call included a non-zero address hint. Unfortunately, clustering never occurred because kern_mmap() always replaced the given address hint when it was zero. So, the ASLR implementation always believed that a non-zero hint had been provided and randomized the mapping's location in the address space. To fix this problem, I'm pushing down the point at which we convert a hint of zero to the minimum allocatable address from kern_mmap() to vm_map_find_min(). Reviewed by: kib MFC after: 2 weeks Differential Revision: https://reviews.freebsd.org/D40743
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@ -1981,14 +1981,14 @@ SYSCTL_INT(_vm, OID_AUTO, cluster_anon, CTLFLAG_RW,
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"Cluster anonymous mappings: 0 = no, 1 = yes if no hint, 2 = always");
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static bool
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clustering_anon_allowed(vm_offset_t addr)
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clustering_anon_allowed(vm_offset_t addr, int cow)
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{
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switch (cluster_anon) {
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case 0:
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return (false);
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case 1:
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return (addr == 0);
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return (addr == 0 || (cow & MAP_NO_HINT) != 0);
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case 2:
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default:
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return (true);
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@ -2111,7 +2111,7 @@ vm_map_find(vm_map_t map, vm_object_t object, vm_ooffset_t offset,
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} else
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alignment = 0;
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en_aslr = (map->flags & MAP_ASLR) != 0;
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update_anon = cluster = clustering_anon_allowed(*addr) &&
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update_anon = cluster = clustering_anon_allowed(*addr, cow) &&
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(map->flags & MAP_IS_SUB_MAP) == 0 && max_addr == 0 &&
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find_space != VMFS_NO_SPACE && object == NULL &&
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(cow & (MAP_INHERIT_SHARE | MAP_STACK_GROWS_UP |
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@ -2255,6 +2255,10 @@ vm_map_find_min(vm_map_t map, vm_object_t object, vm_ooffset_t offset,
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int rv;
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hint = *addr;
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if (hint == 0)
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cow |= MAP_NO_HINT;
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if (hint < min_addr)
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*addr = hint = min_addr;
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for (;;) {
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rv = vm_map_find(map, object, offset, addr, length, max_addr,
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find_space, prot, max, cow);
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@ -383,6 +383,7 @@ long vmspace_resident_count(struct vmspace *vmspace);
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#define MAP_CREATE_STACK_GAP_DN 0x00020000
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#define MAP_VN_EXEC 0x00040000
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#define MAP_SPLIT_BOUNDARY_MASK 0x00180000
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#define MAP_NO_HINT 0x00200000
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#define MAP_SPLIT_BOUNDARY_SHIFT 19
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@ -353,10 +353,12 @@ kern_mmap(struct thread *td, const struct mmap_req *mrp)
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* the hint would fall in the potential heap space,
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* place it after the end of the largest possible heap.
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*
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* There should really be a pmap call to determine a reasonable
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* location.
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* For anonymous mappings within the address space of the
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* calling process, the absence of a hint is handled at a
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* lower level in order to implement different clustering
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* strategies for ASLR.
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*/
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if (addr == 0 ||
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if (((flags & MAP_ANON) == 0 && addr == 0) ||
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(addr >= round_page((vm_offset_t)vms->vm_taddr) &&
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addr < round_page((vm_offset_t)vms->vm_daddr +
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lim_max(td, RLIMIT_DATA))))
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